Adsorptive Removal of Fluoroquinolone Antibiotics

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Sustainability 2015, 7, 12947-12957; doi:10.3390/su70912947 OPEN ACCESS

sustainability ISSN 2071-1050 www.mdpi.com/journal/sustainability Article

Adsorptive Removal of Fluoroquinolone Antibiotics Using Bamboo Biochar Yanbin Wang 1, Jian Lu 2,*, Jun Wu 3,*, Qing Liu 2, Hua Zhang 2 and Song Jin 4 1 2

3 4

Zhejiang Forestry Academy, Hangzhou 310023, China; E-Mail: [email protected] Key Laboratory of Coastal Environmental Processes and Ecological Remediation, Yantai Institute of Coastal Zone Research, Chinese Academy of Sciences, Yantai 264003, China; E-Mails: [email protected] (Q.L.); [email protected] (H.Z.) Qinghai Institute of Salt Lakes, Chinese Academy of Sciences, Xining 810008, China Department of Civil and Architectural Engineering, University of Wyoming, Laramie, WY 82071, USA; E-Mail: [email protected]

* Authors to whom correspondence should be addressed; E-Mails: [email protected] (J.L.), [email protected] (J.W.); Tel.: +86-535-2109-278 (J.L.); Fax: +86-535-2109-000 (J.L.). Academic Editor: Zhiyong Jason Ren Received: 28 July 2015 / Accepted: 17 September 2015 / Published: 22 September 2015

Abstract: The occurrence of fluoroquinolone antibiotics in wastewater has drawn great attention. Adsorption of widely used fluoroquinolone antibiotics (enrofloxacin and ofloxacin) in wastewater using bamboo biochar was investigated. More than 99% of fluoroquinolone antibiotics were removed from the synthetic wastewater through adsorption. Adsorption capacities of bamboo biochar slightly changed when pH increased from 3.0 to 10.0. The adsorption capacity of bamboo biochar increased sharply when the initial concentration of enrofloxacin or ofloxacin increased from 1 to 200 mg L−1 and then began to plateau with further increases in initial concentration. The maximum adsorption capacity (45.88 ± 0.90 mg·g−1) was observed when the ratio of bamboo biochar to fluoroquinolone antibiotics was 10. The enrofloxacin adsorption capacity of bamboo biochar decreased from 19.91 ± 0.21 mg·g−1 to 14.30 ± 0.51 mg·g−1 while that of ofloxacin decreased from 19.82 ± 0.22 mg·g−1 to 13.31 ± 0.56 mg·g−1 when the NaCl concentrations increased from 0 to 30 g·L−1. The adsorptions of fluoroquinolone on bamboo biochar have isotherms that obeyed the Freundlich model (r2 values were in the range of 0.990–0.991). Keywords: fluoroquinolone; antibiotics; wastewater; bamboo biochar; adsorption

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1. Introduction The wide and frequent use of antibiotics in aquaculture has resulted in the development and spreading of antibiotic resistance [1]. Antibiotics kill or inhibit bacteria [2], which can subsequently lead to their persistence in biological wastewater treatment process [3]. An alternative method investigated for antibiotics removal from wastewater is the adsorption technique [4]. Activated carbon adsorption has been recognized as the most effective method, producing good-quality effluents that have low concentrations of dissolved organic compounds in wastewater from pharmaceutical manufacturing [4]. Due to the high operational cost of the traditional activated carbon adsorption techniques, many low cost adsorbents have been used to remove various pollutants [5–8]. Among these adsorbents, bamboo biochar has shown promising application for the removal of organic pollutants [5]. Bamboo biochar has approximately ten times more surface area and four times more sorption capacity than char [9]. Fluoroquinolone antibiotics have been widely useful in controlling many of the bacterial pathogens [1,10,11]. Studies have reported that fluoroquinolone antibiotics may be genotoxic [12]. The presence of fluoroquinolone antibiotics in the environment is potentially linked to their resistant bacteria in various environments, which may result in disturbed aquatic ecosystems and make humans and animals more susceptible to antibiotic resistant microbes [13]. The occurrence of fluoroquinolone antibiotics in various environments has drawn great attention [1,13]. Little information is available on the adsorption removal of fluoroquinolone antibiotics using bamboo biochar. In this study, removal of enrofloxacin and ofloxacin, which are widely used fluoroquinolone antibiotics, was investigated using bamboo biochar. The objective of this study was to evaluate the feasibility of using bamboo biochar as a cost-efficient adsorbent for removing fluoroquinolone antibiotics from wastewater. The impacts of pH, initial concentration, bamboo biochar dose, and ion strength on the adsorption were also investigated. The final goal was to obtain initial information on the treatment of wastewater containing fluoroquinolone antibiotics at low cost using bamboo biochar as the adsorbent. 2. Materials and Methods 2.1. Chemicals, Bamboo Biochar, and Synthetic Aquaculture Wastewater Standards of enrofloxacin and ofloxacin (purity of 98.5%) were obtained from Sigma-Aldich. Deionized (DI) water was purified using a MilliQ Plus system (Millipore, Bedford, MA, USA). All other reagents used were of reagent grade. Bamboo biochar was prepared from bamboo sawdust by pyrolysis at 500 °C. Before the experiments, bamboo biochar was thoroughly washed with distilled water five times to remove the residual acids, Si, and soluble salts, and then oven-dried for 24 h at 105 °C. To make the synthetic wastewater containing fluoroquinolone antibiotics, pond water was filtered using Whatman GF/D glass fiber filters (pore size: 0.45 μm, Fisher Scientific, Pittsburgh, PA, USA) and spiked with enrofloxacin and ofloxacin, which are widely used fluoroquinolone antibiotics. 2.2. Adsorption Test The initial concentration of enrofloxacin or ofloxacin was 100 mg·L−1, except the study on the effect of initial concentration on adsorption. To estimate the applicability of bamboo biochar as an adsorbent

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for fluoroquinolone antibiotics, 50 mg of bamboo biochar was placed in the sealed flasks those contained 10 mL of water with fluoroquinolone antibiotics. The flasks were shaken at a rolling speed of 170 rpm under a constant temperature of 25 ± 2 °C. After 96 h, the bamboo biochar was separated from solution by centrifugation (12000 rpm) for 5 min. The supernatant was collected for HPLC analysis. All experiments were performed in duplicate. The pH of the water was not adjusted using 1 N HCl or NaOH, except in the study on the effect of pH on fluoroquinolone antibiotics adsorption. The experiment has two controls. In the case of bamboo sawdust control, bamboo biochar was replaced by bamboo biochar. In the case of natural attenuation control, the unfiltered pond water without adsorbent was spiked with fluoroquinolone. To better understand the effect of pH on the adsorption process, the pH of the wastewater was adjusted to 2.0, 3.0, 4.0, 5.0, 6.0, 7.0, 8.0, 9.0, 10.0, and 11.0 before mixing with the bamboo biochar. To understand the effect of initial concentration on adsorption, synthetic wastewater with different initial concentrations of enrofloxacin or ofloxacin ranging from 1 to 500 mg·L−1 was prepared and then mixed with bamboo biochar. To understand the effect of bamboo biochar dose on the adsorption, 10 mL of synthetic aquaculture wastewater was mixed with different bamboo biochar doses. The bamboo biochar addition amounts ranged from 10 to 100 mg, causing the related ratios of bamboo biochar to fluoroquinolone antibiotics to range from 10 to 100. To determine the effect of ion strength on the adsorption, NaCl (0, 0.3, and 3 g·L−1) was added into the synthetic wastewater (original salinity was 0.09 g·L−1) to achieve salinities of 0.09, 0.39, and 3.09 g·L−1. 2.3. Adsorption Kinetics Experiment To gain insight into the effect of contact time on fluoroquinolone antibiotics adsorption, samples were collected at 0, 0.5, 1, 2, 4, 8, 12, 24, 48, 72, and 96 h. Three typical kinetics models (including pseudo first-order, pseudo second-order, and Elovich models) were applied to understand the adsorption kinetics of fluoroquinolone antibiotics on bamboo biochar according to [14]. The equations are shown below:

ln(qe − qt ) = ln qe − k1 ⋅ t

(1)

t 1 1 = + ⋅t 2 qt k2 ⋅ qe qe

(2)

qt = a + b ln t

(3)

where t is the adsorption time; qt is adsorption capacity at time t; qe is the saturated adsorption capacity; k1 is the rate constant of psuedo first-order adsorption; k2 is the rate constant of psuedo second-order adsorption; and a and b are Elovich rate constants. 2.4. Adsorption Isotherm Experiment The relationship between adsorption and aqueous concentration at a given temperature is described as adsorption isotherm [14]. For these isotherm experiments, the initial concentrations of enrofloxacin or ofloxacin were 10, 20, 30, 40, 50, 60, 70, 80, 90, and 100 mg L−1. The experiment was conducted at 25 °C. The commonly used adsorption isotherm models are the Freundlich isotherm model and the Langmuir isotherm model [14]. The Langmuir equation (Equation (4)) and Freundlich equation (Equation (5)) are shown as follows [14]:

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1 1 1 = + q eq βQm C eq Qm

(4)

1 ln qeq = ln K f + ln C eq n

(5)

where qeq is the adsorption capacity per unit mass adsorbent; Ceq is the adsorbate concentration in aqueous solution; Kf is the Freundlich coefficient; n is the Freundlich empirical constant; Qm is the adsorption capacity under monolayer adsorption; and ß is the surface adsorption affinity constant. 2.5. Analysis Methods Enrofloxacin and ofloxacin were analyzed using an Agilent 1200 high performance liquid chromatography (HPLC) with a DAD and a Hypersil BDS column (250 mm × 4.6 mm, 5 μm) following previous methods [15,16] with slight modification. The mobile phase was a 10:15:75 (in volume) mixture of methanol, acetonitrile and acidified water (1% formic acid) with a flow rate of 1.0 mL min−1. UV detector wavelengths were set at 280 nm. The injection volume was 50 μL. The column temperature was 35 °C. The amount of adsorbed enrofloxacin or ofloxacin qt at different time t, was calculated as follows:

qt =

(C 0 − C t )V m

(6)

where C0 and Ct are the initial concentration of enrofloxacin or ofloxacin and the concentration at time t, respectively; V stands for the volume of solution; and m is the mass of adsorbent. The maximum biodegradation rate of enrofloxacin or ofloxacin was determined from the time course of the removal, using points in the linear portion of graphs that released enrofloxacin or ofloxacin concentration to time. Specific surface area and pore structure were determined using a Brunauer-Emmet-Teller (BET)-N2 surface area analyzer (ASAP 2000, Micromeritics, Norcross, GA, USA) using nitrogen adsorption/desorption isotherm at 77 K [17,18]. Surface areas of the samples were measured based on the BET method [19]. Barrett-Joyner-Halendar (BJH) theories were used for the analysis of pore size distributions [18]. 3. Results and Discussion

3.1. Adsorptive Removal of Fluoroquinolone Antibiotics Using Bamboo Biochar The rapid removal of enrofloxacin and ofloxacin was observed. The maximum removal rate for enrofloxacin and ofloxacin were 81.91 ± 0.41 and 88.17 ± 0.35 mg·L−1·h−1, respectively, which were much higher than those (

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